And I'd also try the permatex again after sanding/filing the holes lightly and then cleaned with pure acetone.

If everything fails you still have options but it gets more complicated. One is simply replacing the boxes with regular fcs plugs or the fusion box. If you're handy you can do it yourself or otherwise andy surf or windsurf repair shop can handle it. Possible you could even get ahold of the new slot boxes and convert the board so that it can also be ridden as a twin, should you want that.

But try the simple (permatex again) solution first. I'll try to ask around and see if other have had this problem and how they solved it.

I've got a 2010 quad with a stripped fcs fin hole, (read earlier post).
Haven't tried the permatex or jb weld products as I don't think they will last long. Such fine thread on the grub screws with metal to plastic is not such a good idea, especially if they are removed often.
The slot box is a much better idea.
I ended up using a slightly larger diameter grub screw with a wider thread. I drilled out the fcs fin box slightly and then tapped the hole.Seems to be working and the fin hasn't come loose as often.
Cheers